backports: Reimplementations of Functions Introduced Since R-3.0.0

Implementations of functions which have been introduced in
R since version 3.0.0. The backports are conditionally exported which
results in R resolving the function names to the version shipped with R (if
available) and uses the implemented backports as fallback. This way package
developers can make use of the new functions without worrying about the
minimum required R version.